The present invention relates to the separation and processing of crude oil. More specifically the present invention relates to the separation and processing of crude oil to maximize the production of gasoline.
The first step in processing almost all petroleum or crude oil is to distill the oil to remove the major portion of the more volatile components and leave a residual oil or residuum ofter referred to as a reduced crude, a topped crude and by various other terms depending upon the specific separation technique. The process by which this is accomplished is often referred to as stripping, skimming, topping or the like and normally involves fractionation of the volatile components into the various preliminary classifications of products. Such fractionation is accomplished in a pipe still or fractionating tower which separates fractions or cuts according to their boiling range. In one specific instance the fractionator is operated at atmospheric pressure and steam is used as a stripping medium.
Generally this preliminary classification or separation follows a somewhat similar pattern for all crude oils. Specifically, a gas is taken off as an overhead, which ultimately may be treated, separated and utilized, in part, as feed for the production of chemicals, a light blending stock of gasoline, etc. and a gasoline fraction, usually referred to as straight run or virgin gasoline is separated as a top side cut from the fractionator and, of course, is useful as an automotive fuel, usually after upgrading as by reforming to higher octane product. From this point it is possible to produce any number of side cuts having different boiling ranges depending upon the type of crude and the ultimate products desired. However, in general, the next lighter side cut is usually a kerosene fraction utilizable as a furnace oil, jet fuel or the like, then a middle distillate often referred to as virgin or straight run gas oil which may be used as a source of lube oil and/or waxes or as a cracking stock for the production of gasoline, depending upon the type of crude oil processed. Finally, all crudes produce a bottoms cut, heretofore referred to as a reduced crude or topped crude, which may be processed to produce asphalt, lube oils, wax products, and the like or at least a portion thereof can be cracked to produce additional gasoline.
The present invention is directed to the separation of a crude oil in an atmospheric tower with stripping steam, the production of a straight run gas oil, which is ultimately cracked, and cracking of the residual crude oil. While in some cases the virgin or straight run gas oil and the residual crude oil are cracked in a single cracking operation, it is preferable to separately crack the virgin gas oil and the residual crude oil since the virgin gas oil is more refractory than the residual crude oil and is best cracked under more severe conditions, particularly a higher temperature than the cracking of the residual crude oil.
To the extent that the crude oil contains significant amounts of sulfur, it is the usual practice to desulfurize the residual crude oil before subjecting the same to cracking.
Irrespective of whether desulfurization is practiced it is also common practice to further separate the residual crude oils into a light gas oil fraction is fed to a cracking step along with the virgin gas oil and a heavier reduced crude which is fed to the less severe cracking step. Such separation, preliminary to cracking, has the additional advantage that the light gas oil fed to the more severe cracking step is essentially freed of contaminants, particularly asphaltic materials, and such contaminants are retained in the reduced crude which is subjected to less severe cracking conditions.
The separation of the residual crude oil can be practiced as a two-step operation in which the residual crude oil is passed to a first fractionating step wherein it is separated into a light gas oil and a heavy crude oil and the heavy crude fraction to a second separation step where a clean gas oil is removed and combined with the virgin gas oil and the light gas oil from the first fractionator and fed to the clean oil or high severity cracker. The reduced crude from the second separator, containing the major portion of the contaminants, is then fed to the dirty oil or less severe cracker. In accordance with the present invention, the second separation step is carried out in a vacuum tower wherein vacuum is drawn on the top of the tower, clean vacuum gas oil is removed as an upper side cut and the vacuum reduced crude is withdrawn as a bottoms product. In such a vacuum tower operation a dirty gas oil fraction may be withdrawn from a lower trapout tray above the locus of the feed to the tower and recycled back to the vacuum tower at a point below the point of introduction of the feed.
Where the residual crude oil is desulfurized, particularly where hydrodesulfurization is utilized, the desulfurized residual crude oil can be fed directly to the first fractionating step without intermediate heating since the oil is heated during the desulfurization step.
While the two-step separation of residual crude oil, in which the second separation step involves vacuum distillation, produces larger quantities of cracked gasoline than other techniques it has been discovered, in accordance with the present invention, that problems still exist which prevent the production of maximum quantities of cracked gasoline. For example, contaminants, such as asphaltic materials, are often carried over with the vacuum gas oil and result in contamination of the catalyst in the high severity cracker as well as other operational problems. It has also been found that significant quantities of light gas oil are retained in the vacuum reduced crude and are thus sent to the less severe cracking step. This results in insufficient cracking of the lighter gas oils and thus a substantial reduction in the volume of cracked gasoline ultimately produced. It has also been found that lack of control over the nature of the residual crude oil from the crude oil stripping step also reduces the ultimate volume of gasoline which can be produced. Further, and related to such imprecise control of the residual crude oil fraction, is the lack of control over the amount of stripping steam employed. This of course often results in a waste of stripping steam and ultimately a waste of energy in a crude oil stripping step.
It is therefore an object of the present invention to provide an improved process for the separation and processing of crude oil which overcomes the above-mentioned and other problems.
Another object of the present invention is to provide an improved method and apparatus for separating and processing crude oil.
Yet another object of the present invention is to provide an improved method and apparatus for the separation and processing of crude oil wherein increased volumes of gasoline are produced.
A still further object of the present invention is to provide an improved method and apparatus for separating crude oil wherein the production or virgin gas oil is increased.
A further object of the present invention is to provide a method and apparatus for separating crude oil wherein the volume of residual crude oil is reduced.
Yet another object of the present invention is to provide an improved method and apparatus for the separation of crude oil wherein the energy requirements are reduced.
A further object of the present invention is to provide an improved method and apparatus for the separation of crude oil wherein a thermal property of residual crude oil is controlled.
Another object of the present invention is to provide an improved method and apparatus for separating crude oil wherein the energy requirements for such separation are reduced and a thermal property of the residual crude oil produced is controlled.
A further object of the present invention is to provide an improved method and apparatus for the separating and processing of crude oil wherein the volume of virgin gas oil is increased, the volume of residual crude oil is decreased and the volume of gasoline produced is increased.
A still further object of the present invention is to provide an improved method and apparatus for separating and processing a residual crude oil.
Another and further object of the present invention is to provide an improved method and apparatus for separating and processing a residual crude oil wherein the volume of gasoline produced is increased.
These and other objects and advantages of the present invention will be apparent from the following description.